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hunting pressures

i installed a new three ton r22 evaporator today to replace one that was leaking refrigerant.i pumped the unit down,installed the coil,new drier,pressure tected it, evacuated it,and restarted the system.when i restarted the unit the pressures seemed to go up and down.the coil has a number 73 piston in it,and the drier was insatlled,about two feet from the evaporator.the suction pressure went from a high of 90 to a low of 60.the head pressure went from a high of 300 to a low of 200.the pressures did not change fast as it took 5 minutes for them to climb up and then back down.there are no low ambient controls on this system.straight ac for comfort cooling.i first thought because of the high load on the evaporator (84 db) that the pressures would settle down after a bit,but after a half hour,thye unit was still operating like this.it was almost like a txv hunting,but this is a fixed orfice system.the head,and sustion pressure went up and down together,so i don't think it is a restriction.the airflow is constant across the evaporator (psc motor,not ecm) the superheat at the compressor went from a low of eight to a high of twenty while this was going on,and the subcooling went from 12 to 20.the unit is cooling so i went ahead and left it online.any ideas as to could be what is causing this hunting condition on a fixed orfice system? it almost seems that the flow rate is exceeding the fixed orfice's capacity and refrigerant is backing up at the enterance to the evaporator,but that is a reach.any assistance would be much appricated.thank you.


  • Empire_2
    Empire_2 Member Posts: 2,343
    edited July 2012
    OK first things first?

    The system you worked on had an evap. leak and it was replaced.  Was any other contractor there before that may have encountered the same problem and added ref?  I am basically drawing attention to the integrity of the charge itself.  Assume there is non-condensables present and remove entire charge to start from scratch.  I understand that you pumped down the system, made the evap change and put back on line.  Since fixed restrictor works on pressure differential and to check the charge (WB temps will calculate proper charge, not DB) you got some funky readings.  I would start over since you don't know who, what, when or anything else so your charge may be the problem.  No need to speculate further since nobody knows the state of the system before you serviced it.  Recover the charge, evac., pressure test, verify component match then add your R-22.  You'll pull you hair out for no reason if you cant be sure there are no non-cond. in system among other things.  Check it out and check back with results.  You can only assume the charge of existing system had no issues before you worked on it.  It's your baby now so start over.  Now if you checked the system before the leak was repaired, hope you noted operation...?  If not, it's a guess.  There may have benn other problems besides the leak.

    Mike T.
  • Techman
    Techman Member Posts: 2,144

    Can you verify the liquid line is full at the metering device? Sight glass or electronic sight glass?
  • don_9
    don_9 Member Posts: 395

    I would bet money that u have some trash at the piston.However it not out of the realm of possibility that the indoor fan could be cycling on and off as well.

    non condensible are going to sit at the top of the condenser coil and not travel around the system it will just reduce your coil capacity and give way to higher pressure but they will be constant not up and down.

    After running the system for 15 mintues shut the system down and check your pt chart your guage readings should match your pt chart if there are non condensibles.

    And has mike mention you may have to start with some virgin refrigerant.
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